Among vehicle-mounted internal combustion engines, some are known to have a variable valve actuation mechanism which varies the performances, including the timings (opening and closing) and the lifts, of engine valves, such as intake valves and exhaust valves for the purpose of enhancing the power output of or improving the cleanness of emissions from the engine. In such internal combustion engines, the variable valve actuation mechanism is controlled on the basis of the operating state of the engine, and the actuation of the engine valves is adjusted to be optimal for the given operating state of the engine. This adjustment serves to enhance the power output of or improve the cleanness of emissions from the internal combustion engines.
For instance, when the internal combustion engine is required to develop a high output, the actuation of the engine valves is so adjusted as to enhance the intake charging efficiency of the engine. By adjusting the actuation of the engine valves in this way with a view to enhancing the intake charging efficiency, combustion can be accomplished in a state in which the combustion chambers are filled with the largest possible volumes of mixture gas, and the power output of the internal combustion engine is thereby enhanced.
Where the power requirement of the internal combustion engine is not so high, the actuation of the engine valves is so adjusted as to maximize the internal EGR amount within a range that does not adversely affect the combustion. Thus, as the internal EGR amount changes in accordance with the valve overlap amount, the actuation of the engine valves is so adjusted as to realize a valve overlap which would give the maximum value of the internal EGR amount within the range described above. By increasing the internal EGR amount to the practicable maximum in this way, the generation of nitrogen oxides (NOx) is restrained even as the combustion temperature drops, resulting in improved emission contents of the internal combustion engine with respect to NOx.
Incidentally, in another internal combustion engine whose valves are made variable in actuation, the actuation of intake valves and that of exhaust valves are separately varied with a view to even more effective adjustment of the internal EGR amount to enhance the intake charging efficiency and improve emission cleanness. For instance, the Japanese Laid-Open Patent Publication No. 11-218035 discloses a configuration in which a variable valve actuation mechanism for varying the timing of intake valves and a variable valve actuation mechanism for varying that of exhaust valves are provided, the valve timings of both intake valves and exhaust valves can be varied by separately controlling these variable valve actuation mechanisms.
In this internal combustion engine, the target valve timing of the intake valves and that of the exhaust valves are separately computed according to the operating state of the engine. Then, the driving of the two variable valve actuation mechanisms are so controlled that the valve timings of the intake valves and of the exhaust valves reach the respective target valve timings separately computed. By adjusting the valve timings of both the intake valves and the exhaust valves in this way, the adjustment of the internal EGR amount to enhance the intake charging efficiency and improve emission cleanness to increase the output of the internal combustion engine can be made even more effective.
When the valve overlap amount is to be controlled to achieve the optimal amount corresponding to the operating state of the internal combustion engine in which the valve timings both the intake valves and the exhaust valves, the target valve timing of the intake valves and that of the exhaust valves are set to such values as enable the valve overlap to achieve the optimal amount mentioned above. Then, the valve timing of the intake valves and that of the exhaust valves are varied to their respective target timings. Such changes in the valve timings of the intake valves and of the exhaust valves eventually cause the valve overlap amount converge on its optimum (hereinafter referred to as the target valve overlap) to bring the valve overlap amount to its optimum.
However, when in the process of changing the valve timings of the intake valves and the exhaust valves to their respective target valve timings, delays in response to the changes would result in a deviation of the amount of the valve overlap from the target valve overlap. For instance, it is conceivable that, from a state in which the valve timings of the intake valves and the exhaust valves are converged on their respective target valve timings, the target valve overlap is varied by a change in the operating state of the engine or any cause, and that, to obtained the changed target valve overlap, the respective target valve timings of the two valves are changed. In this case, from the time the valve timings of the intake valves and the exhaust valves begin to change toward their respective target valve timings until they converge on the target timings, the valve overlap amount is off the target valve overlap.
There may also emerge a situation in which, while the valve timings of the intake valves and the exhaust valves are being changed according to the operating state of the engine either in the advancing direction or the delaying direction, the valve overlap amount is adjusted to the target valve overlap. In such a situation, the valve overlap amount may be made too great or too little by the difference between the response speed when the valve timing of the intake valves is varied and the response speed when the valve timing of the exhaust valves is varied. The difference in response speed between changes in the valve timings of the intake valves and the exhaust valves is due to the circumstance that, even if the same variable valve actuation mechanisms are used on the intake valve side and the exhaust valve side, it is unavoidable for the oil feed paths used for driving the two mechanisms to differ in length and for this difference to give rise to the aforementioned difference in response speed.
If any difference in the speed of response to changes in valve timing occurs between the intake valves and the exhaust valves when the valve timings of both are to be advanced delayed to the target valve timings, the valve overlap amount once varies in the direction away from the target valve overlap. Hereupon, with respect to each of the situations stated in [I] through [IV] below, the variations of the valve overlap amount away from the target valve overlap will be described below.
[I] Where the valve timings of both the intake valves and the exhaust valves are to be advanced to the target valve timing and the valve overlap amount is to be decreased toward the target valve overlap.
In this case, the valve timing of the intake valves is moved in the direction of increasing the valve overlap amount (advanced) while the valve timing of the exhaust valves is moved in the direction of decreasing the valve overlap amount (advanced), the valve overlap amount is thereby brought closer to the target valve overlap. However, if the response speed of advancing the valve timing of the intake valves (in the direction of increasing the valve overlap amount) is faster than that of advancing the valve timing of the exhaust valves (in the direction of decreasing the valve overlap amount), the valve overlap amount will once expand and vary in the direction away from the target valve overlap. As a consequence, in the process of varying the valve timings, the valve overlap amount will greatly deviate from the target valve overlap.
[II] Where the valve timings of both the intake valves and the exhaust valves are to be advanced to the target valve timing and the valve overlap amount is to be increased toward the target valve overlap.
In this case, as in [I], the valve timings of the intake valves and the exhaust valves are varied. However, if the response speed of advancing the valve timing of the intake valves (in the direction of increasing the valve overlap amount) is slower than that of advancing the valve timing of the exhaust valves (in the direction of decreasing the valve overlap amount), the valve overlap amount will once shrink and vary in the direction away from the target valve overlap. As a consequence, in the process of varying the valve timings, the valve overlap amount will greatly deviate from the target valve overlap.
[III] Where the valve timings of both the intake valves and the exhaust valves are to be delayed to the target valve timing and the valve overlap amount is to be decreased toward the target valve overlap.
In this case, the valve timing of the intake valves varies in the direction of decreasing the valve overlap amount (delayed) and the valve timing of the exhaust valves varies in the direction of increasing the valve overlap amount (delayed), thereby bringing the valve overlap amount closer to the target valve overlap. However, if the response speed of delaying the valve timing of the intake valves (in the direction of decreasing the valve overlap amount) is slower than that of delaying the valve timing of the intake valves (in the direction of increasing the valve overlap amount), the valve overlap amount will once expand and vary in the direction away from the target valve overlap. As a consequence, in the process of varying the valve timings, the valve overlap amount will greatly deviate from the target valve overlap.
[IV] Where the valve timings of both the intake valves and the exhaust valves are to be delayed to the target valve timing and the valve overlap amount is to be increased toward the target valve overlap.
In this case, as in [III], the valve timings of the intake valves and the exhaust valves are varied. However, if the response speed of delaying the valve timing of the intake valves (in the direction of decreasing the valve overlap amount) is faster than that of delaying the valve timing of the intake valves (in the direction of increasing the valve overlap amount), the amount of the valve overlap will once shrink and vary in the direction away from the target valve overlap. As a consequence, in the process of varying the valve timings, the valve overlap amount will greatly deviate from the target valve overlap.
As described above, if the valve overlap amount deviates from the target valve overlap in the process of altering the valve timings of the intake valves and the exhaust valves toward the target valve timing, the following troubles will occur regarding the operating state of the engine.
If the valve overlap amount deviates from the target valve overlap in the increasing direction, the internal EGR amount may become too large and bring down the combustion temperature unduly or the blow-by of intake from the intake passage to the exhaust passage may increase to destabilize combustion. Moreover, if the internal EGR amount becomes too large, gases not contributing to combustion out of the gases present in the combustion chambers during combustion may excessively increase with a corresponding decrease in oxygen volume, inviting combustion in an oxygen-deficient condition and accordingly an increase in HC emissions from the internal combustion engine.
On the other hand, if the valve overlap amount deviates from the target valve overlap in the decreasing direction, the internal EGR amount will become too small. As a consequence, a drop in combustion temperature due to internal EGR makes the temperature insufficient for restraining the generation of NOx, inviting an increase in NOx emission from the internal combustion engine. Furthermore, the deviation of the valve overlap amount from the target valve overlap in the decreasing direction leads to a drop in the fuel efficiency of the internal combustion engine.
Incidentally, the above-discussed troubles regarding the state of engine operation ensuing from the deviation of the valve overlap amount from the target valve overlap is generally true of the type of internal combustion engine in which the valve overlap amount is adjusted by separately varying other valve actuation than the valve timings of valves, such as the lifts of intake valves and exhaust valves.